Use references to mutable state in commands

[jwilger]

This pull request includes several changes to the mneme Rust project, focusing on updating method signatures and improving state handling. The most important changes include modifying the set_state method to take a reference and updating the apply method to return a reference to self.

Version update:

• Cargo.toml: Updated the version of the mneme package from 0.4.2 to 0.5.0.

Method signature updates:

• src/command.rs: Changed the set_state method in the Command trait to take a reference to Self::State.
• src/lib.rs: Updated the set_state methods in test implementations to take a reference to Self::State. [1] [2] [3]
• tests/test_cases.rs: Modified the set_state methods in various command implementations to take a reference to Self::State. [1] [2] [3] [4]

State handling improvements:

• src/command.rs: Updated the apply method in the AggregateState trait to return a reference to self and modified the implementation for the unit type.
• src/lib.rs: Changed the apply method in the StatefulCommandState implementation to return a reference to self.
• tests/test_cases.rs: Updated the apply method in the StatefulCommandState implementation to return a reference to self.